A gate valve located at the end of a pipe that is $1500m$ long is suddenly opened. The pipe is connected to a reservoir, and the exit of the pipe is located at a height of $20m$ below the water level. The diameter of the pipe is $d=0\mathrm{.}9m,$ and the friction factor through the pipe can be assumed to be $f=0\mathrm{.}025.$ Calculate the time after the sudden opening of the valve for the velocity to reach $90\%$ of its final velocity. Figure $1$ shows the reservoir.

Figure $1$: Reservoir

$[20]$